BIOLOGICAL FACTORS 



207 



Europe, and rabbits, introduced into Australia, increased to 

 enormous numbers in only a few years. 



Natural communities are made up of groups of species adapted 

 to living together. The numbers and sizes of individuals are de- 

 termined by the entire complex of environmental factors. If a 

 species is eliminated, others of the community may increase and 

 take its place, or there may then be opportunity for an incidental 

 species to become a part of the community. Usually, if a species is 

 introduced, it does not reproduce and gradually dies out. Occa- 

 sionally, an introduced species has the necessary characteristics to 

 compete successfully and to reproduce regularly. Then adjust- 

 ments must be made within the community and a new balance 

 among its members must be established. Such a species might even 

 become a dominant, and then the adjustments would result in a 

 new community. The prickly pear (Opuntia inermis), introduced 

 in Australia, became a dominant and made useless more than thirty 

 million acres in Queensland alone. 



FIG. 100. Massed water hyacinth covering the water in Louisiana swamp- 

 land. The dusting by airplane is part of an experimental eradication program. 

 -Courtesy of Department of Wildlife and Fisheries, Louisiana. 



